[WIP] Misleading error reporting when token not available: Error reported to user as "model not found"

[Copilot]

Thanks for assigning this issue to me. I'm starting to work on it and will keep this PR's description up to date as I form a plan and make progress.

Original issue description:

When the plugin encounters an authentication issue, this may be shown to the user as "Model not found ". Only opening the logs shows the real issue - Example:

[WARN  Fr 04 Jul 2025 11:39:24 CEST] /home/masssmde/.local/share/nvim/lazy/CopilotChat.nvim/lua/CopilotChat/client.lua:396: Failed to authenticate with copilot: ...zy/CopilotChat.nvim/lua/CopilotChat/config/providers.lua:67: Failed to find GitHub token
[WARN  Fr 04 Jul 2025 11:39:24 CEST] /home/masssmde/.local/share/nvim/lazy/CopilotChat.nvim/lua/CopilotChat/client.lua:354: Failed to authenticate with copilot: ...zy/CopilotChat.nvim/lua/CopilotChat/config/providers.lua:67: Failed to find GitHub token
[WARN  Fr 04 Jul 2025 11:39:24 CEST] /home/masssmde/.local/share/nvim/lazy/CopilotChat.nvim/lua/CopilotChat/client.lua:354: Failed to authenticate with github_models: ...zy/CopilotChat.nvim/lua/CopilotChat/config/providers.lua:67: Failed to find GitHub token
[ERROR Fr 04 Jul 2025 11:39:24 CEST] /home/masssmde/.local/share/nvim/lazy/CopilotChat.nvim/lua/CopilotChat/init.lua:913: ...re/nvim/lazy/CopilotChat.nvim/lua/CopilotChat/client.lua:440: Model not found: o3-mini
[ERROR Fr 04 Jul 2025 11:39:32 CEST] /home/masssmde/.local/share/nvim/lazy/CopilotChat.nvim/lua/CopilotChat/init.lua:913: ...re/nvim/lazy/CopilotChat.nvim/lua/CopilotChat/client.lua:440: Model not found: o3-mini

Consider elevating "token not found" warning to an error?

Fixes #1179.

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